The present invention relates to a method of repairing metallic parts, and in particular to a method of repairing superalloy turbine blades.
Over the years, superalloy materials have been developed to provide mechanical strength to turbine blades and vanes operating at high temperatures. Most modem high temperature superalloy articles such as nickel-based, precipitation strengthened superalloys used in the manufacture of rotating gas turbine blades are complex alloys at the cutting edge of high temperature metallurgy. No other class of alloys can match their high temperature strength. As these turbine blades are difficult and expensive to manufacture, it is far more desirable to repair a damaged blade than to replace one. As a result, a variety of repair methods have been developed and reported, such as for example conventional fusion welding, plasma thermal metal spraying, brazing etc. These processes are most suitable for providing relatively thin coatings. Traditional weld repair has met with only limited success. The quantities of certain precipitate-forming elements (mainly aluminum and titanium) that are added specifically for high temperature strength are primarily responsible for the poor welding record. The weldability of blade superalloys is limited principally by their tendency to form cracks. Two distinct types of cracking have been identified: (1) hot cracking and (2) strain age cracking (SAC). Hot cracking occurs in the filler metal and heat, affected zone (HAZ) during welding and is typically in the form of tiny fissures, or microcracks, beneath the surface of the weldment. Strain age cracking occurs during post weld heat treatment, usually initiating in the HAZ and often propagating well into the adjacent base alloy. Strain age cracks are generally much longer than hot cracks, sometimes extending several inches into the base material.
Weld filler materials that have been most effective in the repair of precipitation strengthened blade superalloys are simpler, solid-solution strengthened alloys that have significantly lower strength. The use of low strength filler materials significantly limits the locations on the blade where weld repairs can be allowed. Current industry practice permits welding only in areas of very low stress, thus, some 80 to 90 percent of blade surfaces are non-repairable. Blades with non-repairable damage are generally returned to suppliers as scrap for credit against replacement blades. The financial impact on utilities is considerable. A single air-cooled, row 1 rotating blade may cost up to thirty-five thousand dollars to replace and, depending upon the turbine manufacturer and model, there are approximately 90 to 120 blades in a typical row.
Various studies have been conducted by the Assignee of the present invention and others to evaluate methods for the repair of precipitation strengthened blade alloys. These studies have included evaluations of both narrow and wide-gap brazing, gas tungsten arc welding, plasma transferred arc welding, and electron beam welding. Narrow-gap brazing techniques have been plagued by joint contamination that results in incomplete bonding, even when elaborate thermochemical cleaning processes precede the brazing operation. Narrow gap brazing also lacks the ability to restore damaged or missing areas on the blade. Joints formed using wide gap brazing methods can be difficult to set-up and porosity in the deposited filler material continues to be a concern.
Traditional Gas Tungsten Arc Welding (GTAW) and Plasma Tungsten Arc Welding (PTAW) are the methods most commonly used in blade repair today, however, they require the use of lower strength fillers in order to avoid cracking (as discussed above).
Many experts believe that low energy welding processes have the highest potential for advancing the state of the art for blade repair. The use of such processes have been shown to reduce cracking. Laser beam welding (LBW) and electron beam welding (EBW) are both low energy processes capable of providing small volume welds with narrow heat affected zones. EBW, however, has inherent limitations in weld path flexibility and must be performed in a vacuum chamber. EBW is currently being used for the repair of gas turbine stationary vanes, combustion components and shaft seals where the joint geometry is relatively straight or in one plane. Application of EBW in the repair of complex blade airfoil shapes would require significant development and is not considered practical at this time. In view of the foregoing, it would be highly desirable to provide an improved technique for repairing metallic parts, such as superalloy turbine blades.
According to the invention there is provided a method of repairing a metallic member, such as a superalloy turbine blade. The blade is prepared by stripping the protective coatings from the blade. The blade is then pre-conditioned for welding by a first hot isostatic process. Once the blade conditioning sequence is complete, the blade is welded using a laser welding technique and by adding weld fillers to the weld area. After the welding step, the blade is sealed by a second hot isostatic process treatment performed at conditions similar to the first hot isostatic process. The blade is finally prepared for re-entry into service.
This repair methodology provides a means to extend the current limits of repair to the more highly stressed areas of the blade. One of the advantages of the technique of the invention is that the use of precipitation strengthened filler alloys more closely matches the mechanical properties of the base alloy. Another advantage of the invention is that the use of Nd:YAG (Yttrium Aluminum Garnetxe2x80x94Doped with Nd) or Carbon Dioxide lasers as the welding heat source, as opposed to conventional arc welding processes, produces smaller heat affected zones and reduces the stress field due to the lower quantity of heat introduced in the weld zone. A further advantage of the invention is that the introduction of a dual hot isostatic process, which brackets the welding application, preconditions the substrate for welding and eliminates any microcracking inherent with the superalloy blades after welding.